1. Field of the Invention
The present invention relates to a buckle apparatus employed in a seatbelt system designed to protect an occupant of a vehicle in an emergency situation of the vehicle.
2. Description of the Related Art
A typical conventional buckle apparatus for use in a seatbelt system is arranged such that an occupant can be fastened by an occupant restraining webbing by engaging a tongue plate attached to an end or intermediate portion of the webbing with the buckle apparatus.
There are various types of buckle apparatus. One of them is a latch type buckle apparatus in which, as shown in FIG. 13, a tongue plate 12 is engaged with a lock member 10 which has its two ends respectively supported by members 14 which constitute the apparatus body. This latch type buckle apparatus has a large tongue plate holding strength and hence extremely high engagement reliability. In this buckle apparatus, disengagement of the tongue plate 12 is effected by sliding the lock member 10 upwardly as viewed in FIG. 13.
In this type of buckle apparatus the tongue plate disengaging force, that is, the force F1 required to slide the lock member 10 when loaded, is represented by the following formula (1): EQU F1=(.mu.1+.mu.2)W.apprxeq.2.mu.W (1)
where
W: webbing tension (load) PA0 .mu.1: coefficient of friction between the lock member 10 and the members 14 .mu.2: coefficient of friction between the lock member 10 and the tongue plate 12 PA0 (it is assumed that .mu.1=.mu.2=.mu.)
Accordingly, the buckle apparatus of the type described above requires a large force for disengaging the tongue plate when a large tension is acting on the webbing.
Another type of conventional buckle apparatus is a prop type buckle apparatus in which, as shown in FIG. 14, a lock member 16 is pivotally supported by a member 18 which constitutes the buckle apparatus body, and a projection 16A which projects from the lock member 16 engages with a tongue plate 12. This type of buckle apparatus requires a relatively small force F2 for disengaging the tongue plate 12 as represented by the following formula (2): EQU F2=.mu..times.W=.mu..times.W/cos.theta.=.mu./cos.theta..times.W
where if it is assumed that .theta.=16.degree. EQU F2.apprxeq.1.04.mu.W.apprxeq.F1/2 (2)
However, this prop type buckle apparatus has the disadvantage that the tongue plate holding strength is lower than that of the latch type buckle apparatus.